Optical communications systems utilize optical carrier waves to transmit communications signals among various items of optical equipment that are coupled into the systems and that use the systems to communicate with one another. The systems utilize optical fiber cables for transmitting the carrier waves from one item of optical equipment to another. For example, an optical communications system may comprise a computer central processing unit, CPU, as one item of equipment, a workstation, a peripheral, such as a printer, and optical fiber cables linked among the CPU, the workstation and printer.
Each item of optical equipment is coupled to the system by means of an optical connector that is constructed for disconnect coupling with the optical fiber cables. A duplex communications system utilizes two optical cables, one for receiving optical signals from an item of optical equipment, and another for sending signals. Each item of optical equipment has an optical emitter for sending the signals and a detector for receiving the signals. Optical connectors provide disconnect coupling for both the optical emitter and the optical detector.
Testing of such items of optical equipment is a necessity to assure proper design. In testing, the environment must accurately simulate the anticipated environment of operation which includes attenuation which comes about through loss of photons by the light signal during transit, thus reducing amplitude.
Fiber attenuation may arise from two sources; absorption and scattering. Impurities in glass absorb light energy, turning photons into heat. Some impurities remain as residue in the glass fiber after purification and processing; others are dopants added purposely to obtain certain optical properties. Scattering results from imperfections in the fibers and from the basic structure of the fiber. Rayleigh scattering comes from the atomic and molecular structure of the glass, and from density and composition variations that are natural by-products of manufacturing. Unintentional variations in density and fiber geometry occur during fiber manufacture and cabling. Small variations in the core diameter, microbends, and small incongruities in the core-to-cladding interface cause loss.
Attenuation for fiber is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from about one dB/km for premium small core glass fibers, to over 2000 dB/km for large core plastic fibers.
Some installed systems involve several miles of optical fibers. Hence, and typically, testing is done by simulation whereby the emitter and detector of the item of optical equipment to be tested are connected to a device that simulates the optical system, and the operation of the item is tested as though the item were coupled into the system itself and not the testing device.
First simulators were devices capable of generating special test signals. Testing was performed externally on the item of equipment being tested. However, as items of optical equipment became increasingly complex, the need for testing has become greater. Further, with the development of more sophisticated capabilities, optical equipment has been designed with internal testing capabilities permitting self-testing. With self-testing, the expense of specialized testing equipment and associated testing procedures has been substantially reduced. In place of long lengths of cabling to simulate actual operations, and in place of simulators that are devices that produce complex signals or measurements, are simplified simulators having internal attenuating mechanisms.
The present invention relates to such simulators and in the form of loop-back attenuators. Loop-back attenuators are defined as simulators providing a communication signal path that forms a loop from the emitter to a detector of the same item of optical equipment such that optical signals transmitted from the item under test are looped back to the same item and internally transmitted among its component parts. Simulators which are loop-back attenuators that purposely simulate a loss of signal intensity expected of a communications system in which the item may be installed for "on-line" operation. Vastagh, U.S. Pat. No. 4,736,100, discloses a known loop-back simulator involving an optical fiber cable formed in a loop and having ends of the fiber connected with alignment ferrules. The loop is installed in an alignment fixture that aligns the ends of the loop with the emitter and detector of the item to be tested.
This known loop-back attenuator suffers from disadvantages. Firstly, it is difficult to provide, within the short loop-back cable structure of the simulator, an attenuation accurately simulating that of the much larger optical system that the optical item, such as a transceiver, will be attached to during actual operation. Another problem is one of magnitude in that too much undiminished optical power may saturate the detector of the transceiver. Another problem is accurately duplicating the amount of attenuation in the operations system so that the testing device creates an environment approximating the operation of the actual system for meaningful test results.
Objects of the present invention include providing a simulator in the nature of a loop-back attenuator that, in a compact device, is capable of reproducing the total attenuation of a substantially larger cable network. Other objects include providing a device capable of sufficiently attenuating optical power between emitter and detector of a transciever or the like, to prevent saturation of the detector, and providing a device which easily and accurately may be controllably altered to match the particular amount of attenuation desired to simulate actual environmental operating conditions or to meet manufacturer's standards.
Another problem is that devices of different manufacturers, and even, indeed, the same devices of the same manufacturer, have emitters that put out differing optical power. An objective in this respect, is to provide a device which may easily be altered to approximate the differing optical power outputs of devices and the attenuation characteristics to be expected in the operating systems.
Another objective of the present invention is providing an optical simulator which is a loop-back attenuator which may be precisely customized to provide a specified or targeted attenuation.